Generally, a transmission for a vehicle employing a single clutch inevitably involves gear shift shocks, delays in acceleration, and the like due to interruption of driving force when shifting gears. In a construction machinery, an agriculture vehicle, or the like with large running resistance and small speed energy, it becomes stopped as soon as driving force is interrupted when shifting gears and may be hard to shift the gears.
To this, it is known that a twin-clutch transmission involves no interruption of driving force to prevent gear shift shocks or delays in acceleration.
However, there is a problem that the twin-clutch transmission is complicated in structure and heavy in weight.
In contrast, a seamless-shift transmission draws attention as one capable of reducing weight.
FIG. 29 is an operationally explanatory view of a seamless-shift transmission. In FIG. 29, for ease of explanation, a shift between a first speed and a second speed will be explained.
The seamless-shift transmission has three first burettes 305 and three second burettes 307 that engage with an input shaft between a first speed gear 301 and a second speed gear 303 and move according to shift operation. On the first and second speed gears 301 and 302, meshing teeth 301a and 303a are formed, and at both ends of the first and second burettes 305 and 305, complicated different faces are formed in front and rear in a rotational direction.
The first and second burettes 305 and 307 are configured to move toward the first speed gear 301 or second speed gear 303 through a spring concerning an operation of a selecting fork.
With such a structure, for example, when shifting into the first speed gear 301, the three first burettes 305 engage with the meshing teeth 301a of the first speed gear 301, and thereafter, the remaining three second burettes 307 engage with the meshing teeth 301a. 
When shifting into the second speed gear, the three second burettes 307 engage with the meshing teeth 303a of the second speed gear 303, and thereafter, the remaining three first burettes 305 engage with the meshing teeth 303a. 
With the first burettes 305 and second burettes 307 having such complicated faces and the selecting operation through the spring, it involves no interruption of driving force to prevent gear shift shocks or delays in acceleration and can reduce the weight.
However, there is a problem that the structure having the first burettes 305, the second burettes 307 and the like is complicated and increases in number of parts.